1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus a position measurement system is used, to determine the position of movable objects such as a substrate table and a patterning device support. The position measurement for the mask and substrate tables has to be performed in all 6 DOF to sub-nanometer resolution, with nanometer accuracy and stability. This is commonly achieved using (single or multi-axis) interferometers to measure displacements in all 6 DOF, with the possibility of redundant axes for additional calibration functions (e.g. calibrations of interferometer mirror flatness on the substrate table).
The performance of an interferometer is generally dependent on the beam length, since the optical pathlength is sensitive to refractive index changes caused by global and/or local environmental changes. Therefore performance of interferometers tends to decrease if beam lengths get longer. The effect of these changes on the performance of the interferometer start to become significant at the accuracy levels required in lithographic devices.
For instance typical problems are noticed due to periodic (high frequent) pressure changes in the environment. These pressure changes have a significant influence on the refractive index of the medium through which the measurement signal goes. It is remarked that it is known to use pressure sensors in order to take these pressure changes into account. However, these pressure sensors are typically too slow to deal with the disturbances. At present no other solution is proven to adequately correct for these pressure changes. Also other environmental factors such as temperature changes, air composition (amount CO2) and such have an influence on the performance of interferometers.
Furthermore, thermal expansion effects of the frame on which the interferometers are arranged, and the substrate table itself, will cause changes in the beam length (physical change of beam path). By requiring (extremely) high specifications on the conditioning of both the environment in which the optical signal of the interferometers travels, and the mechanics that are involved in the measurement loop, these thermal expansion effects can be reduced.